One of devices for changing a frequency characteristic of an inputted audio signal is a graphic equalizer. Generally, in the graphic equalizer, a plurality of adjustment points need to be adjusted one by one. Therefore, there is a problem that the operation burden for a user is large when there are many adjustment points.
For this reason, Japanese Patent Provisional Publications No. H11-112254A (hereinafter, “Patent Document 1”) and No. 2004-62503A (hereinafter, “Patent Document 2”) propose devices that can overcome the above problem. The devices disclosed in Patent Document 1 and Patent Document 2 detect a path, on a screen of an equalizer, traced by a user using a stylus pen or a finger (hereinafter, “touch operation path”), and set a gain for each adjustment point (a central frequency of each band) on the basis of the detected touch operation path. The user can set gains of all the adjustment points by one operation that is by tracing the screen of the equalizer, without operating a plurality of adjustment points one by one separately and independently. However, in the graphic equalizer, since the central frequency of each band and the Q value (an adjustment band width) is fixed, adjustment flexibility is low. Therefore, it is difficult to accurately set a frequency characteristic of a filter (hereinafter, “filter characteristic”) that the user desires using the graphic equalizers disclosed in Patent Document 1 or Patent Document 2.
One of devices similar to the graphic equalizer is a parametric equalizer. In the parametric equalizer, in addition to the gain for the central frequency of each band, the central frequency of each band and the Q value can be adjusted. Japanese Patent Provisional Publication No. H5-175773A (hereinafter, “Patent Document 3”) discloses an example of the parametric equalizer. The parametric equalizer disclosed in Patent Document 3 prepares in advance a plurality of filter coefficient groups of which the Q value changes in proportion to an absolute value of the gain for the central frequency of each band, reads out a filter coefficient group appropriate for a gain adjustment operation by a user from among the plurality of filter coefficient groups prepared in advance, and uses the read out filter coefficient group to adjust a filter characteristic to a gentle and continuous one as a whole. It could be considered that a filter characteristic that a user desires can be accurately set by applying the technique disclosed in Patent Document 3. However, in the parametric equalizer, since ripples remain in the filter characteristic, there is a problem that unnecessary reverberation components are detected when music or the like is played back.